Bisphenols, and in particular bisphenol A (2,2 bis(p-hydroxyphenyl) propane), have become industrially significant reactants for a number of processes including the preparation of polycarbonates. Bisphenols are prepared on an industrial scale by one of two processes: an acid-catalyzed or HC1 process and an ion exchange process, in which an acidic ion exchange resin such as sulfonic acid-substituted polystyrene is employed.
The bisphenol is prepared by condensation of two moles of phenol with one mole of a ketone or aldehyde, for example acetone, in the presence of an acidic catalyst. In addition to the bisphenol, however, the product stream from the reaction includes unreacted phenol, which is included in excess over the stoichiometric requirement, and various isomers of the desired bisphenol and other by-products. Because these by-products can compromise the properties of products made using the bisphenol, they need to be separated. One technique for accomplishing this separation involves cooling the product stream to induce crystallization of a 1:1 bisphenol:phenol adduct which can then be further processed by washing, distillation, extraction and/or steam stripping to produce a purified bisphenol product. Processes for the production and purification of bisphenols are well known, and are described inter alia in U.S. Pat. Nos. 4,107,218; 4,294,994; 5,210,329; 5,243,093; 5,245,088; 5,288,926; 5,368,827; 5,786,522; and 5,874,644.
The crystallization of bisphenol from the reactant stream is generally carried out in a crystallizer which is designed for continuous or semi-continuous operation. The crystallization coolers have a complex interior structure to provide significant surface area for heat exchange between the bisphenol reaction stream and a circulating coolant. Over time, adduct precipitates out on the cooling surfaces on the interior crystallizer, reducing the efficiency of cooling and in some cases plugging orifices in the crystallizer and restricting flow of materials into and out of the crystallizer. When this happens, the crystallizer must be taken off-line for a period of time for cleaning. This interruption of the manufacturing process is costly, and it is therefore desirable to perform the cleaning as infrequently as possible, and with maximum effectiveness.
One method for cleaning the crystallizer and associated coolers is by increasing the temperature on the inside of the crystallizer to melt out deposits of crystals that may have formed. This can be accomplished by increasing the temperature of the heat exchange fluid, or by introducing hot phenol into the interior of the crystallizer. This method, however, is not always effective, and generally requires about 24 hours of down time to complete the cleaning procedure. In addition, this type of process suffers from poor reproducibility, and requires delicate control of the start-up procedure to return the crystallizer to the efficient production of the desired crystallized bisphenol adduct. U.S. Pat. No. 5,856,589 discloses an alternative approach in which phenol containing from 3 to 40 weight percent of water is used to dissolve deposited bisphenol adduct from the cooling and crystallization surfaces. The use of combinations of phenol and water reduces the time required for a melt out and the frequency of required melt outs. Nevertheless, there remains a need for further improvements in cleaning procedures, to further reduce the process down-time associated with such cleaning.